What does the word 'Hallelujah' mean?

Hallelujah comes from two Hebrew words ‘Halelu’ which means ‘everybody praise’ and ‘Yah’ which comes from the word for God. So it means ‘everybody praise God’.

 
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In Old Testament times the word ‘Hallelujah’ was used to tell people to praise God; now it is a word that Christians say when they are praising God.